A spud is a pile that is used by dredging vessels and which engages with the bottom of a water column in order to provide a fixed point for such a dredging vessel during dredging. The spud, also referred to as spud-pile, has a substantial vertical orientation and is subjected to loads, in particular resulting from the dredging process and the movements of the vessel. The vessel may be able to move with respect to the spud. In practice, a spud weighs between a few dozen tons and 200 tons. The spud may be made of steel and may comprise drain holes and holes for receiving locking pins.
The spud may be lowered to engage with the water bottom. This may be done by simply releasing the spud such that it falls to the water bottom. The lower end of the spud may be formed as a sharp end to penetrate the water bottom. Lifting means are provided for lifting the spud.
A plurality of spud guiders may be provided at different heights and are connected to the dredging vessel for accommodating the spud and guiding the spud when it is lowered or lifted. The spud guiders may be provided on the outside of the dredging vessel. For instance, three spud guiders may be provided, while the spud needs to be supported by at least two spud guiders to prevent loosing the spud.
In order to provide the vessel with an improved freedom of movement, the spud guiders may be provided on a spud carriage which can move relative to the hull of the vessel in a horizontal direction.
If the spud is lifted too much, it may reach a position in which it is no longer supported by enough spud guiders. The spud may than exert too much force on the spud guider(s) still supporting the spud, which may damage the spud guiders and may result in falling over of the spud, possibly onto the dredger.
Therefore, monitoring the position of the spud during lifting is needed.
It is noted that the vertical position of the spud is difficult to predict. In the first place the depth of the water bottom may not be known accurately and also it is difficult to know how far exactly the spud has penetrated the water bottom. Also, as the spud may rotate about is longitudinal body axis, locking holes provided in the spud for receiving locking pins may be difficult to trace and are therefore difficult to use to determine the vertical position of the spud.
As the spud is made of steel and the guiding elements and the dredging vessel are usually also made of steel, or at least comprise steel, detection of the position of the spud by detecting the presence or absence of steel is relatively difficult.
NL6905248A and EP2258904A1 describe spud systems known from the prior art. However, these documents do not describe how to accurately detect the presence or absence of steel to prevent lifting the spud too much.
JP-A-56047386 describes a method which relies a plurality of height measurements such as a height between the spud and the sea bottom and a lifting height and performs calculations based on these measurements. This approach is vulnerable to measurements errors, which could result in an erroneous calculated position of the lower end of the spud which could introduce the risk of lifting the spud too much.
JP-A-10310378 describes the use of two photoelectric sensors which monitor shift length of the vertically moved spud by sensing passage of projections and recesses. This device is also vulnerable to measurements errors. Measurement errors may result in an erroneous calculated position of the spud and thus in an erroneous position of the lower end of the spud.